This invention relates to polyvinyl chloride plasticizers which have been derived from vegetable oil, their method of making and their method of use.
Polyvinyl chloride (PVC), the best known and most widely used of the vinyl plastics, is most often utilized in one or both of two general forms: substantially unplasticized PVC, and plasticized PVC. The substantially unplasticized form of PVC, commonly known as rigid PVC, is used for pipework, ducts, and similar applications in which high resistance to chemical substances is required. The plasticized form of PVC is widely useful for numerous applications including films, sheeting, cable coverings, moldings, stationary products, conveyer belting, toys, and hose. Plasticized PVC is also used as a substitute for leather and may be used as a fabric for clothing and various furnishings.
Among the most important physical characteristics of plasticized PVC are flexibility and pliability. These physical characteristics are achieved by compounding PVC resin with one or more materials which serve as plasticizers following their addition to the PVC resin. Broadly defined, plasticizers are high boiling point liquids which are used as ingredients in lacquers and certain plastics such as PVC. These liquids do not evaporate from the matrices they are added to, but rather preserve the flexibility and adhesive power of cellulose lacquer films or the flexibility of plastic sheeting and film. The majority of PVC plasticizers currently utilized in industrial processes are petroleum-derived phthalates and benzoate compounds. Dioctyl phthalate (DOP) and diallyl phthalate (DAP) are examples of petroleum-derived compounds commonly used as primary plasticizers for PVC.
While effective as primary plasticizers, petroleum-derived plasticizers are subject to several significant limitations. In addition to being processed from a nonrenewable source, petroleum-derived PVC plasticizers are often expensive to produce due to fluctuations in the price and availability of crude oil. Furthermore, petroleum-derived plasticizers such as DOP are suspected to disrupt human endocrine activity (see Modern Plastics, January 1998, p35). Therefore, it may be desirable or even necessary to limit the use of petroleum-derived plasticizers in certain situations, especially when the plasticized product comes into human contact at ambient temperature and especially at elevated temperature. Thus, there is a need for a low-cost, non-toxic, environmentally safe alternative to the petroleum-derived plasticizers that are currently incorporated into a great many consumer products. PVC plasticizers which are derived from vegetable oils provide such an alternative.
Unmodified vegetable oils are largely incompatible with polyvinyl chloride resin. However, certain modified derivatives of vegetable oils, such as epoxidized soybean oil, are compatible with PVC resin and provide an alternative to petroleum-based plasticizers. Furthermore, vegetable oils are derived from a renewable source, namely plants, and are not likely to create the potential for physiological disturbances or other injuries to persons coming into contact with products which require plasticizers in their composition.
Epoxidized soybean oil is currently used to a limited extent as a secondary plasticizer and co-thermal stabilizer in the processing and manufacturing of flexible, semi-rigid and rigid PVC products. Thermal stabilizer are needed in PVC formulations because at typical extrusion processing temperatures, PVC polymer undergoes slow elimination of hydrochloric acid because of labile beta hydrogen atoms. This leads to formation of isolated double bonds followed by fast allyl activated dehydohalogenation resulting in the formation of dark colored conjugated polyenes. This degradation, if not controlled, is auto-catalytic. Epoxidized soybean oil has been found useful in the prior art as a stabilizer in typical PVC formulations at a level of 2-3%, but increased thermal stabilization has been determined to be up to 27 pph PVC. To further impart high temperature stability, metal soaps such as zinc and calcium stearates are included as thermal stabilizers in combination with epoxidized soybean oil in PVC resins.
Despite the usefulness of epoxidized soybean oil as a thermal stabilizer, at use levels of about 5% (and in some cases up to 15%) in the compounded PVC resin, ESO tends to exude due to poor compatibility with the resin matrix at high levels. This incompatibility makes partially esterified and epoxidized soybean oil largely unsuitable as a primary plasticizer for use in PVC resins in high concentrations because primary plasticizers often represent up to 50% of the PVC matrix. The poor compatibility of partially esterified and epoxidized soybean oil with PVC resin is presumably due to factors such as the non-optimized match of the solubility parameter of partially epoxidized soybean oil polarity with that of PVC. Thus, for soybean oil or other vegetable oil derivatives to be useful as primary plasticizers, a useful oil must be modified to significantly improve its compatibility with PVC resins. Importantly, any chemical modifications should not significantly diminish the thermal stabilizing properties of a preferred vegetable oil.
Polyol esters have been used in PVC compositions as disclosed, for example, in U.S. Pat. No. 4,421,886 issued to Worschel et al., which discloses PVC compositions wherein the partial ester of pentaerythritol with fatty acids is used in combination with a polybasic lead compound as a stabilizer/lubricant combination. U.S. Pat. No. 4,605,694 issued to Walker discloses a plasticized PVC composition which includes a trimellitate ester, and a pentraerythritol alkanoic acid ester. U.S. Pat. No. 5,886,072 issued to Linskey et al. discloses a flame retardant composition which includes a pentaerythritol ester plasticizer used with PVC resin along with other plasticizers. U.S. Pat. No. 5,430,108 issued to Schlosberg et al. also discloses a plasticized PVC composition which includes a pentraerythritol alkanoic acid ester with mixed C5, C7 and C9 alkanoic acids.
The present invention provides unique combinations of plasticizers formed with substantially fully esterified and substantially fully epoxidized fatty acids from the fatty acids derived from commonly available vegetable oils.
Accordingly, these and other disadvantages of the prior art are overcome by the present invention which provides plasticizers derived from vegetable oil which are highly compatible with polyvinyl chloride resin, and as such are excellent for use as primary plasticizers in PVC resins. Typically, vegetable oil-based plasticizers can be used in PVC resins only at levels of 15% or less. The modified vegetable oil-based plasticizers of the present invention are significantly better than commercially available vegetable oil-based plasticizers, such as the commercially available ESO, and may be used at levels above 15%. Other embodiments provide for levels of plasticizers above about 20%. A typical upper limit for plasticizer materials of the present invention is about 50% although higher levels up to 70% can be used. It is expected that the higher concentrations of plasticizers with high epoxy content will provide additional thermal stabilization. Thus the materials of the invention provide for high and effective levels plasticizer for PVC formulations without evaporating or exuding from the PVC matrix. Furthermore, certain embodiments of the present invention exhibit both comparable plasticizing performance, and reduced migration tendencies as compared to the current industry standard, dioctyl phthalate (DOP), which is derived from petroleum feed stock. The plasticizers of the present invention are also effective co-thermal stabilizers of PVC resins, and are particularly effective when combined with conventional metal soaps or other commercially available thermal stabilizers.
In a broad embodiment, the improved vegetable oil-based plasticizers of the present invention are made by a preferred method which includes the basic steps of (i) creating ester linkages by attaching fatty acids derived from vegetable oils (e.g., oleic, linoleic, linolenic acid, and palmitoleic acid, etc.) to monoalcohols (monools) or polyalcohols (polyols) by means of direct esterification; and (ii) epoxidizing the esterified products (which contain saturated or unsaturated fatty acids) from step (i) to increase the polarity and increase the solubility parameter of these reaction products close to the solubility parameter of PVC. Presumably, increasing the polarity and solubility parameters increases the compatibility of the vegetable-oil based plasticizer with polyvinyl chloride resin. Alternatively, the first step of this broad method (direct esterification) may be substituted with the step of transesterification, wherein a monool or polyol reacts directly with the vegetable oil to produce the desired product plus glycerin, and wherein a monool or polyol reacts with the lower alkyl ester of vegetable oil acid to produce the desired product plus the lower alcohol. Typically the saturated and unsaturated fatty acids are distributed randomly on each molecule of a polyol that is esterified with the fatty acids. This process also results in a random mix of esterified fatty acids.
An alternative broad embodiment includes the steps of (i) interesterification of one ester with another ester, or of a vegetable oil such as soybean oil with another vegetable oil such as linseed oil; and (ii) subsequent epoxidation of the product of the interesterification reaction. Interesterified oil may be further reacted with alcohols (monools and polyols) by transesterification of the interesterified product, followed by epoxidation of the transesterified, interesterified product. The immediately above mentioned alternative may also be used here.
The modified vegetable-oil based plasticizers of the present invention are derived from soybean oil, and include: (i) epoxidized pentaerythritol tetrasoyate; (ii) epoxidized propylene glycol disoyate; (iii) epoxidized ethylene glycol disoyate; (iv) epoxidized methyl soyate; (v) epoxidized sucrose octasoyate; and (vi) the epoxidized product of soybean oil interesterified with linseed oil.
Therefore it is an object of the present invention to provide vegetable oil-based plasticizers which are useful as primary plasticizers for vinyl plastics, and which can completely replace petroleum-based compounds, such as DOP, as the primary plasticizers used with vinyl plastics.